The present invention relates to an apparatus for controlling shifts in an automatic transmission.
An automatic transmission for vehicles generally effects shifts by switching the engaged/released states of frictional engagement units such as clutches or brakes in accordance with changes in running conditions such as a vehicle speed or a degree of throttle opening. Moreover, one-way clutches are used to facilitate shift controls and smoothen the shifts.
One example of the automatic transmission of this kind is shown in skeleton diagram in FIG. 19. This automatic transmission is disclosed in Japanese Patent Laid-Open No. 4742/1989 and equipped with two sets of planetary gear mechanisms, a plurality of frictional engagement units and a pair of one-way clutches arrayed in series with each other. Specifically, a carrier 3 of a first planetary gear mechanism 1 and a ring gear 4 of a second planetary gear mechanism 2 are connected to each other, and a ring gear 5 of the first planetary gear mechanism 1 and a carrier 6 of the second planetary gear mechanism 2 are connected to each other. Moreover, an input shaft 7 is arranged along the center axis of those planetary gear mechanisms 1 and 2. A first clutch C1 is interposed between the input shaft 7 and a sun gear 8 of the first planetary gear mechanism 1, and a second clutch C2 is interposed between the input shaft 7 and the carrier 6 of the second planetary gear mechanism 2. A third clutch C3 is interposed between the input shaft 7 and a sun gear 9 of the second planetary gear mechanism 2. Between the carrier 6 and the sun gear 9 of the second planetary gear mechanism 2, on the other hand, there are interposed a fourth clutch C4 and a first one-way clutch F1, which are arrayed in series with each other. The first one-way clutch F1 is engaged if the rotating direction of the clutch drum of the fourth clutch C4, i.e., the rotating direction of the sun gear 8 with the fourth clutch C4 being engaged is forward (i.e., in the same direction as that of the input shaft 7) with respect to the carrier 6. Between the clutch drum of the fourth clutch C4 and a casing 10, there is interposed a second one-way clutch F2 which is engaged when the clutch drum is rotated backward (i.e., in the direction opposite to that of the input shaft 7).
On the other hand, the brake means is exemplified by both a first brake B1, i.e., a band brake fixed on the outer circumference of the clutch drum of the fourth clutch C4 and a second brake B2 for braking the rotations of the ring gear 5 of the first planetary gear mechanism 1 and the carrier 6 of the second planetary gear mechanism 2, which are connected to each other. Thus, the first brake B1 stops the rotations of the carrier 6 and the sun gear 9 of the second planetary gear mechanism 2 selectively through the fourth clutch C4 or the first one-way clutch F1.
In the gear train shown in FIG. 19, individual gear stages of four forward and one reverse stages are set by engaging the engagement units such as the clutches and/or brakes in combinations, as shown in FIG. 20. In FIG. 20: symbols .largecircle. indicate the engaged states; symbols .circleincircle. indicate the engaged states to be taken at the time of engine braking; and blanks indicate the released states.
The automatic transmission equipped with the gear train shown in FIG. 19 ordinarily performs an upshift from 2nd to 3rd speeds, for example, in the following manner. An electronic control type automatic transmission is equipped with a shift diagram using the throttle opening and the vehicle speed as parameters so that a shift signal is outputted to engage the second clutch C2 at the instant when the throttle opening and the vehicle speed vary to cause the running state to cross the 2-3 shift curve.
This upshift is established in either the so-called "power-on upshift", in which the throttle opening or the engine output is increased to accelerate the vehicle speed, or the so-called "power-off upshift" in which the throttle opening is decreased to lower the engine output. If, at the time of the latter power-off upshift, the shift signal is outputted to engage the second clutch C2 simultaneously as the change in the running state from the 2nd to 3rd speeds is detected, an inertial torque is established to give the passenger a discomfort.
In the example shown in FIG. 19, the 2nd speed is set by engaging the first clutch C1 and the fourth clutch C4 and accordingly by engaging the second one-way clutch F1. In this state, the sun gear 8 of the first planetary gear mechanism 1 rotates together with the input shaft 7, while the sun gear 9 of the second planetary gear mechanism 2 being fixed, so that the carrier 3 of the first planetary gear mechanism 1 and the ring gear 4 of the second planetary gear mechanism 2 rotate integrally with the drive gear 11 or the output member at a lower speed than the input shaft 7. And, the carrier 6 of the second planetary gear mechanism 2 and the ring gear 5 of the first planetary gear mechanism 1 connected to the second clutch C2 rotate at a lower speed than the drive gear 11.
If the second clutch C2 is engaged immediately after the decision of the shift to the 3rd speed at the 2nd speed state, the ring gear 5 of the first planetary gear mechanism 1 and the carrier 6 of the second planetary gear mechanism 2 are abruptly accelerated from their forward low speeds to a speed equal to that of the input shaft 7. This abrupt acceleration appears as a positive torque in the accelerating direction on the output shaft so that a torque in the opposite direction that of the expected torque is temporarily established and felt as the discomfort at the time of a deceleration (or power-off) in which the throttle opening is decreased. In other words, the shift shocks are deteriorated.
These phenomena are illustrated in FIG. 21. If an upshift from the 2nd to 3rd speeds is decided at an instant t.sub.0 while the vehicle is running at the 2nd speed and if a predetermined solenoid valve is switched, the torque capacity begins to be built up in the second clutch C2 at an instant t.sub.1 which is inevitably delayed. Since, at this instant, the engine speed (more specifically, turbine speed) is not sufficiently decreased yet, a positive torque appears to cause the passenger feel the shift shocks. Thus, the turbine speed is decreased as a result of such increase in the load, and the output torque drops to a negative value at an instant t.sub.2.
In order to decrease the shift shocks, on the other hand, the switching timings of engagements/releases of the frictional engagement units are controlled in the prior art in accordance with the drive state of the engine. This control is exemplified in Japanese Patent Laid-Open No. 76967/1990 by a shift control apparatus in which the clutch engaging rates for a downshift causing the rotating members connected to the input shaft to change are made different for the driving (or power-on) time and the driven (or power-off) time. Specifically, the engaging rate is increased at the power-on downshift time by feeding an oil pressure to the clutches through two orifices having larger and smaller effective areas but is decreased at the power-off downshift time by feeding an oil pressure to the clutches only through the orifice having the smaller effective area. As a result, the shifting timing reflects the drive state of the engine so that the shift shocks can be softened.
In the apparatus disclosed in Japanese Patent Laid-Open No. 76967/1990, however, the timings for engaging the clutches at the downshift time can be changed, but their controls are carried out by changing the orifices for passing the oil pressure. As a result, the control range is not wide but may fail to match the actual running condition. Specifically, the running states at the time of a shift such as the engine output, the running speed, the viscosity of oil or the gear stage to be set are remarkably various, and the control of switching the oil pressure passing orifices between only two kinds is difficult to optimum the engaging or releasing timings of the frictional engagement units for all the running conditions. At the power-on downshift time, for example, the timings for engaging the frictional engagement units may possibly be so premature as to establish a negative output torque, which will be felt as the shift shocks.